The present invention relates to magneto-optic rotators, which rotate the direction of polarization of polarized light passed through them, and particularly to magneto-fiber optic rotators utilizing the Faraday effect.
Some substances are themselves optically active in a manner which rotates the polarization of light passed therethrough. However, glass (silica), the material most commonly used for optical fibers, does not exhibit this property. In 1845, Faraday discovered that glass and other substances, otherwise devoid of this property, acquire it when placed in a strong magnetic field. Light traversing the substance, parallel to the lines of magnetic force, is affected thereby, so that the direction of polarization of the light is rotated. This is commonly referred to as the "Faraday effect". A particularly interesting feature of the Faraday effect is that the direction of rotation for a given lightwave is the same from the viewpoint of a fixed observer, regardless of the direction of propagation. Thus, a light beam passing through the field, once in each direction, would have its rotation doubled (which is not the case with natural optical activity).
The amount of such Faraday rotation is dependent upon the following factors:
1. The Verdet constant of the material (an indication of the susceptibility of the material to the Faraday effect); PA0 2. The intensity of the portion of the magnetic field applied to the material that is parallel to the direction of propagation of the light; and PA0 3. The length over which the magnetic field is applied to the material.
Silica has a relatively low Verdet constant. Thus, in order to generate an appreciable Faraday rotation in a silica optical fiber, the magnetic field intensity must be extremely strong, or the length of fiber immersed in the field must be extremely long, or both. As an indication of the difficulties involved in achieving significant Faraday rotation in glass optical fibers, it will be understood that a single mode fiber approximately 12 centimeters in length will require a magnetic field strength (in a direction parallel to the fiber) of approximately 1,000 gauss to produce a Faraday rotation of only about 11/2 degrees. Thus, because of the low Verdet constant of silica, it is very difficult to produce substantial Faraday rotation in glass optical fibers.